Oxide semiconductors are of prime interest as alternatives to amorphous silicon TFT channel materials for the use in active matrix panels for display applications. In particular, the high field-effect mobility and low threshold voltage compared to amorphous silicon make oxide semiconductors attractive alternatives. One of the most promising oxide materials is amorphous indium-gallium-zinc-oxide (a-IGZO, or simply IGZO, which is InGaO3(ZnO)5) that has been shown to have superior electrical properties compared to amorphous silicon and no uniformity drawbacks that are typically found for polycrystalline Si. IGZO has high mobility, electrical stability, and excellent uniformity. Other transparent conductive films, such as indium tin zinc oxide (ITZO) (actually, amorphous In2O3—SnO2—ZnO (a-ITZO)) have been applied particularly in the field of thin film displays in TFTs, as the active layer. Since such amorphous oxide semiconductor films, in particular IGZO and ITZO, can be formed on a polymer film at relatively low temperatures, they have been applied to light-weight portable electronic items using such thin film displays.
There are four basic TFT structures, including bottom-gate staggered, top-gate staggered, bottom-gate coplanar and top-gate coplanar. In such a TFT, the source and drain may be formed of a thin metal film, from a metal such as Al or Cu or alloys of these metals. Mo has been used in the past as the source/drain, but for the future devices, the more conductive metals Al and Cu are planned. However, if Al or Cu are applied in direct contact with IGZO or ITZO films, problems can arise, such as diffusion of Cu into the IGZO or ITZO or creation of additional vacancies in the IGZO or ITZO by reaction of the Al with oxygen from the IGZO or ITZO. Thus, in most cases, a thin film of Mo or Ti is provided and the more conductive Al or Cu or alloys thereof is used as the primary conductor. The layer of Mo or Ti separates the Al or Cu from the oxide semiconductor, thus avoiding these problems. This thin metal film is formed on the IGZO or ITZO, thus forming a metal composite layer on the IGZO or ITZO, e.g., of Mo/Cu/Mo/IGZO, Mo/Al/Mo/IGZO, Cu/Mo/IGZO, Mo/IGZO, Al/Mo/IGZO. In the foregoing composites, Ti may replace Mo. To form the TFT structure, a layer of the thin metal film, Mo/Cu/Mo, Mo/Al/Mo, Cu/Mo, Mo, Al/Mo (again, Ti may replace Mo), that has been formed must be etched to form the separate source and drain. In order to etch the thin metal film and not etch away the oxide semiconductor film, it is necessary that the etchant have a high selectivity for the metals relative to the oxide semiconductor film. Dry etchants have been used, but have poor selectivity and tend to damage the underlying oxide semiconductor film. Commonly used wet etchants, such as PAN, have shown poor selectivity for metal(s) relative to the underlying semiconductor oxide films. Other etchants such as ammonia/peroxide, have been found to lack reproducibility, consistency and sufficient selectivity for etching the metal(s) relative to the oxide semiconductor film. The most important problem is the selectivity of the etchant for the “bottom” metal in the stack, e.g., Mo or Ti, with respect to the oxide semiconductor, e.g., IGZO or ITZO, and this problem has remained unsolved.
Thus, there is a continuing need for etchants that can provide reliable and consistent high etching selectivity for the metal(s) relative to the underlying oxide semiconductor film.